Editing Definition of planet (section)

=== Extrasolar planets and brown dwarfs ===
{{main|Extrasolar planet|Brown dwarf}}
The discovery since 1992 of [[extrasolar planet]]s, or planet-sized objects around other stars ({{Extrasolar planet counts|planet_count}} such planets in {{Extrasolar planet counts|system_count}} [[planetary system]]s including {{Extrasolar planet counts|multiplanetsystem_count}} [[List of multiplanetary systems|multiple planetary systems]] as of {{Extrasolar planet counts|asof}}),<ref name="Encyclopaedia">
{{cite encyclopedia
|last1=Schneider |first1=J.
|date=September 10, 2011
|title=Interactive Extra-solar Planets Catalog
|url=https://exoplanet.eu/catalog/
|encyclopedia=[[Extrasolar Planets Encyclopaedia]] |access-date=July 13, 2012
}}</ref> has widened the debate on the nature of planethood in unexpected ways. Many of these planets are of considerable size, approaching the mass of small stars, while many newly discovered brown dwarfs are, conversely, small enough to be considered planets.<ref>{{cite web
 |year=2006 
 |title=IAU General Assembly: Definition of Planet debate 
|url=http://astro2006.meta.mediastream.cz/Astro2006-060822-01.asx
 |archive-url=https://archive.today/20120713105841/http://astro2006.meta.mediastream.cz/Astro2006-060822-01.asx
 |url-status=dead
 |archive-date=July 13, 2012
|access-date=September 24, 2006 }}</ref> The material difference between a low-mass star and a large [[gas giant]] is not clear-cut; apart from size and relative temperature, there is little to separate a gas giant like Jupiter from its host star. Both have similar overall compositions: hydrogen and [[helium]], with trace levels of heavier [[chemical element|elements]] in their [[atmosphere]]s. The generally accepted difference is one of formation; stars are said to have formed from the "top down", out of the gases in a nebula as they underwent gravitational collapse, and thus would be composed almost entirely of hydrogen and helium, while planets are said to have formed from the "bottom up", from the accretion of dust and gas in orbit around the young star, and thus should have cores of [[silicate]]s or ices.<ref>{{cite journal |year= 2004| author=G. Wuchterl| title=Giant planet formation| journal=Institut für Astronomie der Universität Wien| volume=67| issue=1–3| pages=51–65| doi=10.1007/BF00613290|bibcode = 1994EM&P...67...51W | s2cid=119772190}}</ref> As yet it is uncertain whether gas giants possess such cores, though the [[Juno mission|''Juno'' mission]] to Jupiter could resolve the issue. If it is indeed possible that a gas giant could form as a star does, then it raises the question of whether such an object should be considered an orbiting low-mass star rather than a planet.{{Update inline|date=April 2024}}

[[File:Brown Dwarf Gliese 229B.jpg|thumb|left|The brown dwarf [[Gliese 229B]] in orbit around its star]]
Traditionally, the defining characteristic for starhood has been an object's ability to [[Nuclear fusion|fuse]] [[hydrogen]] in its core. However, stars such as brown dwarfs have always challenged that distinction. Too small to commence sustained hydrogen-1<!--deuterium is also hydrogen--> fusion, they have been granted star status on their ability to fuse [[deuterium]]. However, due to the relative rarity of that [[isotope]], this process lasts only a tiny fraction of the star's lifetime, and hence most brown dwarfs would have ceased fusion long before their discovery.<ref>{{Cite journal | last=Basri | first=Gibor | title= Observations of Brown Dwarfs | journal=Annual Review of Astronomy and Astrophysics | year=2000 | volume=38 | pages=485–519 | doi=10.1146/annurev.astro.38.1.485 | bibcode=2000ARA&A..38..485B}}</ref> [[Binary star]]s and other multiple-star formations are common, and many brown dwarfs orbit other stars. Therefore, since they do not produce energy through fusion, they could be described as planets. Indeed, astronomer [[Adam Burrows (astronomer)|Adam Burrows]] of the [[University of Arizona]] claims that "from the theoretical perspective, however different their modes of formation, extrasolar giant planets and brown dwarfs are essentially the same".<ref>{{cite journal |author1=Burrows, Adam |author2=Hubbard, William B. |author3=Lunine, Jonathan I. |author4=Leibert, James | year=2001 | title=The Theory of Brown Dwarfs and Extrasolar Giant Planets | doi=10.1103/RevModPhys.73.719 | journal=Reviews of Modern Physics | volume=73 | issue=3 | pages=719–765 | arxiv=astro-ph/0103383|bibcode = 2001RvMP...73..719B |s2cid=204927572 }}</ref> Burrows also claims that such stellar remnants as [[white dwarfs]] should not be considered stars,<ref>Croswell p. 119</ref> a stance which would mean that an orbiting [[white dwarf]], such as [[Sirius B]], could be considered a planet. However, the current convention among astronomers is that any object massive enough to have possessed the capability to sustain atomic fusion during its lifetime and that is not a black hole should be considered a star.<ref>{{cite book | author=Croswell, Ken | year=1999 | title=Planet Quest: The Epic Discovery of Alien Solar Systems| publisher= Oxford University Press|page=119|isbn=978-0-19-288083-3}}</ref>

The confusion does not end with brown dwarfs. María Rosa Zapatero Osorio et al. have discovered many objects in young [[star cluster]]s of masses below that required to sustain fusion of any sort (currently calculated to be roughly 13 Jupiter masses).<ref>{{cite journal |year= 2000|author1=Zapatero M. R. Osorio |author2=V. J. S. Béjar |author3=E. L. Martín |author4=R. Rebolo |author5=D. Barrado y Navascués |author6=C. A. L. Bailer-Jones |author7=R. Mundt | title=Discovery of Young, Isolated Planetary Mass Objects in the Sigma Orionis Star Cluster| journal= Division of Geological and Planetary Sciences, California Institute of Technology| doi=10.1126/science.290.5489.103 | volume=290 |issue=5489 | pages=103–107|pmid=11021788 |bibcode = 2000Sci...290..103Z }}</ref> These have been described as "[[rogue planet|free floating planets]]" because current theories of Solar System formation suggest that planets may be ejected from their [[star system]]s altogether if their orbits become unstable.<ref>{{cite journal| last=Lissauer| first= J. J.| title= Timescales for Planetary Accretion and the Structure of the Protoplanetary disk| journal= Icarus| volume= 69| pages=249–265| year=1987| doi=10.1016/0019-1035(87)90104-7| bibcode=1987Icar...69..249L| issue=2| hdl= 2060/19870013947| hdl-access=free}}</ref> However, it is also possible that these "free floating planets" could have formed in the same manner as stars.<ref>{{cite news | title=Rogue planet find makes astronomers ponder theory| agency=Reuters| url=http://edition.cnn.com/2000/TECH/space/10/06/space.planets.reut/index.html| access-date=May 25, 2006 | date=October 6, 2000}}</ref>

[[File:Sol Cha-110913-773444 Jupiter.jpg|thumb|The solitary [[Cha 110913-773444]] (middle), a possible [[sub-brown dwarf]], set to scale against the Sun (left) and the planet [[Jupiter]] (right)]]
In 2003, a working group of the IAU released a position statement<ref>{{cite web|year=2001 |title=Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union |work=[[IAU]] |url=http://www.dtm.ciw.edu/boss/definition.html |access-date=May 25, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060916161707/http://www.dtm.ciw.edu/boss/definition.html |archive-date=September 16, 2006 }}</ref> to establish a working definition as to what constitutes an extrasolar planet and what constitutes a brown dwarf. To date, it remains the only guidance offered by the IAU on this issue. The 2006 planet definition committee did not attempt to challenge it, or to incorporate it into their definition, claiming that the issue of defining a planet was already difficult to resolve without also considering extrasolar planets.<ref>{{cite web|title=General Sessions & Public Talks|url=http://www.astronomy2006.com/media-stream-archive.php|year=2006|publisher=International Astronomical Union|access-date=November 28, 2008|url-status=dead|archive-url=https://web.archive.org/web/20081208140503/http://www.astronomy2006.com/media-stream-archive.php|archive-date=December 8, 2008}}</ref> This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.<ref>{{cite web |title=Official Working Definition of an Exoplanet |url=https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/ |work=IAU position statement |access-date=November 29, 2020 }}</ref> The official working definition of an ''exoplanet'' is now as follows:

{{blockquote|
* Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the [[Lagrange point#Stability|L4/L5 instability]] (M/M<sub>central</sub> < 2/(25+{{math|{{radical|621}}}}) are "planets" (no matter how they formed).
* The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
}}

The IAU noted that this definition could be expected to evolve as knowledge improves.

[[File:hubbledwarf.jpg|left|thumb|upright|CHXR 73 b, an object which lies at the border between planet and brown dwarf]]
This definition makes location, rather than formation or composition, the determining characteristic for planethood. A free-floating object with a mass below 13 Jupiter masses is a "sub-brown dwarf", whereas such an object in orbit around a fusing star is a planet, even if, in all other respects, the two objects may be identical. Further, in 2010, a paper published by Burrows, David S. Spiegel and John A. Milsom called into question the 13-Jupiter-mass criterion, showing that a brown dwarf of three times solar [[metallicity]] could fuse deuterium at as low as 11 Jupiter masses.<ref name=spiegel/>

Also, the 13 Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object.<ref name=spiegel>{{Cite journal|arxiv=1008.5150 |author1=David S. Spiegel|author2=Adam Burrows |author3=John A. Milsom|title=The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets |journal=The Astrophysical Journal|volume=727|issue=1|pages=57|year=2010|doi=10.1088/0004-637X/727/1/57|bibcode=2011ApJ...727...57S|s2cid=118513110}}</ref> As of 2011 the [[Extrasolar Planets Encyclopaedia]] included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around {{Jupiter mass|13|jup=y}} in the observed mass spectrum reinforces the choice to forget this mass limit".<ref>{{cite journal|last1=Schneider |first1=J. |last2=Dedieu |first2=C. |last3=Le Sidaner |first3=P. |last4=Savalle |first4=R. |last5=Zolotukhin |first5=I. |title=Defining and cataloging exoplanets: The exoplanet.eu database| date=2011| volume=532| issue=79| journal=[[Astronomy & Astrophysics]] |arxiv=1106.0586| doi=10.1051/0004-6361/201116713|pages=A79 |bibcode=2011A&A...532A..79S|s2cid=55994657 }}</ref> 
As of 2016 this limit was increased to 60 Jupiter masses<ref>{{Cite book|last=Schneider|first=Jean|date=April 4, 2016|title=Exoplanets versus brown dwarfs: the CoRoT view and the future|chapter=III.8 Exoplanets versus brown dwarfs: The CoRoT view and the future |page=157 |doi=10.1051/978-2-7598-1876-1.c038|arxiv=1604.00917 |isbn=978-2-7598-1876-1 |s2cid=118434022 }}</ref> based on a study of mass–density relationships.<ref>{{cite journal |arxiv=1506.05097|last1= Hatzes Heike Rauer|first1= Artie P.|title= A Definition for Giant Planets Based on the Mass-Density Relationship|year= 2015|doi=10.1088/2041-8205/810/2/L25|volume=810|issue= 2|journal=The Astrophysical Journal|page=L25|bibcode = 2015ApJ...810L..25H |s2cid= 119111221}}</ref> The [[Exoplanet Data Explorer]] includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the [[sin i ambiguity]]."<ref name=eod>{{cite arXiv|eprint=1012.5676v1|author=Wright, J. T.|title=The Exoplanet Orbit Database|class=astro-ph.SR|year=2010|display-authors=etal}}</ref> The [[NASA Exoplanet Archive]] includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.<ref>{{Cite web|title=Exoplanet Criteria for Inclusion in the Exoplanet Archive|url=https://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html|access-date=March 29, 2023|website=exoplanetarchive.ipac.caltech.edu}}</ref>

Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location, is whether the core [[pressure]] is dominated by [[Coulomb barrier|Coulomb pressure]] or [[electron degeneracy pressure]].<ref>{{cite journal |doi=10.1146/annurev.earth.34.031405.125058 |journal=Annu. Rev. Earth Planet. Sci. |volume=34 |title=Planetesimals To Brown Dwarfs: What is a Planet? |pages=193–216 |year=2006 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B|last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |s2cid=119338327 }}</ref><ref>{{cite journal |author1=Boss, Alan P. |author2=Basri, Gibor |author3=Kumar, Shiv S. |author4=Liebert, James |author5=Martín, Eduardo L. |author6=Reipurth, Bo |author7=Zinnecker, Hans |title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ? |journal=Brown Dwarfs |volume=211 |page=529 |year=2003 |bibcode=2003IAUS..211..529B }}</ref>

One study suggests that objects above {{Jupiter mass|10|jup=y}} formed through gravitational instability and not core accretion and therefore should not be thought of as planets.<ref>{{Cite journal|last=Schlaufman|first=Kevin C.|date=January 18, 2018|title=Evidence of an Upper Bound on the Masses of Planets and its Implications for Giant Planet Formation|journal=The Astrophysical Journal |volume=853 |issue=1 |page=37 |doi=10.3847/1538-4357/aa961c|arxiv=1801.06185 |bibcode=2018ApJ...853...37S |s2cid=55995400 |doi-access=free }}</ref>

A 2016 study shows no noticeable difference between gas giants and brown dwarfs in mass–radius trends: from approximately one Saturn mass to about {{Solar mass|0.080 ± 0.008}} (the onset of hydrogen burning), radius stays roughly constant as mass increases, and no obvious difference occurs when passing {{Jupiter mass|13}}. By this measure, brown dwarfs are more like planets than they are like stars.<ref name=ChenKipping>{{cite journal |last1=Chen |first1=Jingjing |last2=Kipping |first2=David |date=2016 |title=Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal=The Astrophysical Journal |volume=834 |issue=1 |page=17 |doi= 10.3847/1538-4357/834/1/17|arxiv=1603.08614 |s2cid=119114880 |doi-access=free |bibcode=2017ApJ...834...17C }}</ref>

====Planetary-mass stellar objects====
The ambiguity inherent in the IAU's definition was highlighted in December 2005, when the [[Spitzer Space Telescope]] observed [[Cha 110913-773444]] (above), only eight times Jupiter's mass with what appears to be the beginnings of its own [[planetary system]]. Were this object found in orbit around another star, it would have been termed a planet.<ref>{{cite web| year=2005| author=Clavin, Whitney| title=A Planet With Planets? Spitzer Finds Cosmic Oddball| work=Spitzer Science Center| url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html| access-date=May 25, 2006| archive-date=October 11, 2012| archive-url=https://web.archive.org/web/20121011011111/http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html| url-status=dead}}</ref>

In September 2006, the [[Hubble Space Telescope]] imaged [[CHXR 73 b]] (left), an object orbiting a young companion star at a distance of roughly 200 AU. At 12 Jovian masses, CHXR 73 b is just under the threshold for deuterium fusion, and thus technically a planet; however, its vast distance from its parent star suggests it could not have formed inside the small star's [[protoplanetary disc]], and therefore must have formed, as stars do, from gravitational collapse.<ref>{{cite web|title=Planet or failed star? Hubble photographs one of the smallest stellar companions ever seen|work=ESA Hubble page|url=http://www.spacetelescope.org/news/html/heic0610.html|year=2006|access-date=February 23, 2007}}</ref>

In 2012, Philippe Delorme, of the [[Institute of Planetology and Astrophysics]] of [[Grenoble]] in France announced the discovery of [[CFBDSIR 2149-0403]]; an independently moving 4–7 Jupiter-mass object that likely forms part of the [[AB Doradus moving group]], less than 100 light years from Earth. Although it shares its spectrum with a [[T dwarf|spectral class T brown dwarf]], Delorme speculates that it may be a planet.<ref>{{cite journal|title=CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus?|author1=P. Delorme |author2=J. Gagn´e |author3=L. Malo |author4=C. Reyl´e |author5=E. Artigau |author6=L. Albert |author7=T. Forveille |author8=X. Delfosse |author9=F. Allard |author10=D. Homeier |journal=Astronomy & Astrophysics|year=2012|arxiv=1210.0305|doi=10.1051/0004-6361/201219984|bibcode=2012A&A...548A..26D|volume=548|pages=A26|s2cid=50935950 }}</ref>

In October 2013, astronomers led by Dr. Michael Liu of the [[University of Hawaii]] discovered [[PSO J318.5-22]], a solitary free-floating [[L dwarf]] estimated to possess only 6.5 times the mass of Jupiter, making it the least massive [[sub-brown dwarf]] yet discovered.<ref name="liu_discovery">
{{cite journal | title = The Extremely Red, Young L Dwarf PSO J318-22: A Free-Floating Planetary-Mass Analog to Directly Imaged Young Gas-Giant Planets | journal = Astrophysical Journal Letters | date = October 1, 2013 |author = Liu, Michael C. |author2= Magnier, Eugene A. |author3= Deacon, Niall R. |author4= Allers, Katelyn N. |author5= Dupuy, Trent J. |author6= Kotson, Michael C. |author7= Aller, Kimberly M. |author8= Burgett, W. S. |author9= Chambers, K. C. |author10= Draper, P. W. |author11= Hodapp, K. W. |author12= Jedicke, R. |author13= Kudritzki, R.-P. |author14= Metcalfe, N. |author15= Morgan, J. S. |author16= Kaiser, N. |author17= Price, P. A. |author18= Tonry, J. L. |author19=  Wainscoat, R. J. | volume = 777 | issue = 2 | doi =10.1088/2041-8205/777/2/L20 | arxiv=1310.0457|bibcode = 2013ApJ...777L..20L | pages=L20| s2cid = 54007072 }}</ref>

In 2019, astronomers at the [[Calar Alto Observatory]] in Spain identified GJ3512b, a gas giant about half the mass of Jupiter orbiting around the red dwarf star [[GJ3512]] in 204 days. Such a large gas giant around such a small star at such a wide orbit is highly unlikely to have formed via accretion, and is more likely to have formed by fragmentation of the disc, similar to a star.<ref>{{cite web|title=Exoplanet discovery blurs the line between large planets and small stars|author=Andrew Norton|date=September 27, 2019|url=https://phys.org/news/2019-09-exoplanet-discovery-blurs-line-large.html|publisher=phys.org|access-date=March 13, 2020}}</ref>